A yarn brake for knitting and spooling machines, as is known from the German Utility Model No. 1 913 720. In this apparatus the braking device is designed in the form of a braking magnet acting upon a metallic yarn winding drum. The magnetic flux of this magnet passes through parts of the drum and is adjustable and controllable. The magnetic field can be influenced either by changing the exciting current of the magnet, which is an electromagnet, or by introducing a metallic element into the air gap between the pole faces of the magnet and the yarn drum. The metallic element more or less diverts the magnetic flux. It is necessary for the yarn drum either to consist completely of a magnetically conductive material such as iron, or to have at least magnetically conductive disk or ring-shaped components to achieve the magnetic braking action. In both cases, the yarn drum will have a relatively big inertia, resulting in a strong flywheel effect because on the one hand magnetically conductive components are essential, and on the other hand a certain minimum diameter is necessary to produce in the area of one drum face a magnetic return for the braking magnets, with a sufficiently high reluctance so that appropriate magnetic induction in the drum components through which the magnetic flux passes.
However, the considerable flywheel effect of the yarn drum renders such a yarn brake useless for all those applications where rapid changes in the speed of yarn travel must be expected. The heavy yarn drum, coupled in a slip-free manner with the moving yarn, cannot follow rapid changes in the speed of yarn travel, with the result that the yarn is either subject to tension peaks or to a collapse in yarn tension, both of which are inadmissible.
Apart from those considerations, the braking effect produced by the stationary braking magnets in the rotational magnetically conductive yarn drum is based on eddy current effects and is very dependent on the speed. Especially at low yarn travel speeds the braking effect drops quickly below an admissible minimum rate.
For practical purposes it is therefore always necessary to go back to disk or ball-type yarn brakes in which the yarn is pulled, frictionally engaged, between two braking surfaces which assert a certain braking force upon it. But with these it is usually unavoidable that sensitive yarns are more or less undesirably affected, while on the other hand the braking effect that can be achieved at low yarn travel speeds is also problematic.
All yarn brakes mentioned above are unable to establish or maintain a definite tension in stationary yarn.